In process automation technology, field devices are often employed, which serve to register and/or influence process variables. For registering process variables, sensors are used, for example, fill-level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH-redox potential measuring devices, electrical conductivity measuring devices, etc., which register the corresponding process variables fill-level, flow, pressure, temperature, pH-value and conductivity. For influencing process variables, actuators are used, for example valves or pumps, via which the flow of a fluid in a section of pipeline or the fill-level in a container can be changed. In principle, all devices, which are employed near to a process and which deliver or work with process-relevant information, are referred to as field devices. A large number of such devices are produced and sold by the Endress+Hauser Group.
In modern industrial facilities, field devices are, as a rule, connected with superordinated units via fieldbus systems (Profibus®, Foundation® Fieldbus, HART®, etc.). Normally, the superordinated units involve control systems or control units, for example a PLC (programmable logic controller). The superordinated units are used, for example, for process control, process visualizing, process monitoring, as well as in the start-up of the field devices. The measurement values registered by the field devices—especially by sensors—are transmitted via the respective bus system to one, or, in given cases, more, superordinated units. Additionally, a transfer of data from the superordinated unit to the field devices via the bus system is necessary, especially for configuring and parametering field devices, as well as for operating actuators.
In addition to a hardwired data transmission between the field devices and the superordinated unit, the possibility of a wireless data transmission also exists. In particular, in the bus systems Profibus®, Foundation® Fieldbus and HART®, a wireless data transmission via radio is specified. Additionally, radio networks for sensors are specified in greater detail in the standard IEEE 802.15.4. For implementing such a wireless transmission of data, newer field devices are in part embodied as radio field devices.
Along with this, there exists the possibility of upgrading field devices without radio units to radio-field devices through the attachment of a wireless adapter featuring a radio unit. The wireless adapter is, in such case, connected to the relevant field device via a fieldbus communication interface of the field device. Via the fieldbus communication interface, the field device can send the data to be transmitted over the bus system to the wireless adapter, which then transmits this via radio to the target location. Conversely, the wireless adapter can receive data via radio and forward such via the fieldbus communication interface to the field device. For example, a wireless adapter by which a conventional field device can be upgraded to a radio field device is described in the publication WO 2005/103851 A1.
Conventional field devices are, as a rule, supplied with electrical power via the bus system, or via lines separately provided for this. If the field device is supplied with energy via the fieldbus, these field devices are then referred to as bus-fed, or two-conductor, devices. In this case, both communication as well as the supplying of energy occurs via a shared 2-conductor-connection. If, in addition to the fieldbus, an additional 2-conductor connection is provided for supplying electrical power, these field devices are then referred to as 4-conductor devices. In the case of the bus-fed field devices, it is especially required that, when they are upgraded to radio field devices through the connection of a wireless adapter, they be supplied with electrical power by the wireless adapter via the fieldbus communication interface. For this purpose, the wireless adapter as a rule includes an electrical current source, for example a disposable battery, a fuel cell, a rechargeable battery, etc. As a rule, two power supply stages are provided in the wireless adapter, wherein a first power supply stage serves to internally supply the system components of the adapter with power, and the second power supply stage serves to externally supply the system components of the field device with power. Both power supply stages are constantly connected to the electrical current source. A power supply stage which serves to the supply the system components of the field device with electrical power is, as a rule, also provided in the field device. The power supply stage of the field device is, in such case, constantly connected to the fieldbus communication interface, and through this, is supplied with electrical power by the wireless adapter.
Since, in the case of such bus-fed field devices, both communication and the supplying of electrical power occurs via the fieldbus communication interface of the field device, corresponding limitations become evident in the case of supplying electrical power. If, for example, the communication interface of the of the field device is constructed according to the HART® bus system—and communication between the field device and the wireless adapter occurs according to the HART® standard—a measured value is transmitted, in an analog manner, by the absolute value of the electrical current. In the case of communication over the fieldbus communication interface, depending on the measured value transmitted, the electrical current value is set to a value between 4 mA and 20 mA (in the following: “Analog measured value transmission according to the 4-20 mA HART® Standard”). In order to assure that the field device is sufficiently supplied with electrical power even in the case of a minimum electrical current value of 4 mA, the voltage is transformed to a correspondingly high value in the second power supply stage of the wireless adapter. In the power supply stage of the field device, the voltage must then once again be transformed to a value corresponding to a voltage required by the individual system components of the field device. A similar problem also becomes evident in the case of the bus systems Profibus® and Foundation® Fieldbus, in the case of which the electrical current flowing over the communication interface is likewise limited.
Through this voltage transformation in the wireless adapter and field device, considerable energy losses are caused. Additionally, a minimum value of electrical power—which is determined by the minimum electrical current value of 4 mA and the voltage which is supplied by the second power supply stage—is delivered by the second power supply stage to the field device. Even if the power requirement of the field device is lower—as is, for example, the case, when a measured value is not recorded by the field device (sensor), when no action is performed by the field device (actuator) or when the field device is in standby mode—the minimum value of electrical power delivered to the field device is nevertheless consumed. In order to lengthen the lifespan of the energy source of the wireless adapter—and thereby also the maintenance intervals—the power consumption of the system (composed of the wireless adapter and the field device) must be reduced. In part for this reason, the field device is shut off during the times when it is not required. This procedure has the disadvantage that the field device runs through a startup mode each time it is turned on for recording a measured value or for performing an action. In such a startup mode, for example, self-checks are performed, capacitors charged, etc. In this way, additional energy is again consumed. A time delay furthermore results thereby.